Generally, the invention relates to an apparatus and a method for graphite furnace atomic absorption spectroscopy. In particular, the invention relates to matrix modifiers for graphite furnace atomic absorption spectroscopy which improve quantitative measurement of metal constituents in unknown samples.
Although there are different approaches to the detailed design and construction of furnace atomizers, all perform the same fundamental process--to generate a population of free analyte atoms so atomic absorption can be measured. In its simplest form, this process is achieved in three stages of heating of the furnace:
1. The drying stage to remove solvent from the sample;
2. An ashing stage to remove organic molecules or inorganic materials; and
3. The atomization stage to generate free analyte atoms within a confined zone coincident with the spectrophotometers optical path.
It is very desirable to produce a well-defined peak of optical absorption during the atomization stage. The height and area of this peak is related to the amount of analyte present in the sample and the conversion efficiency of the atomization. Peak characteristics are presented on a chart recorder or established by electronic peak measurement systems built directly into the spectrophotometer. Such systems typically provide both height and area measurements from one characteristic peak. Concentration values of metal analytes are calculated for unknown samples using comparative measurements with predetermined standards containing those metal constituents.
Theoretically, conversion efficiency of the analyte in the sample into vaporized free metallic molecules whose light absorption is measured by the spectrophotometer should be high because all the available sample is used to produce the atom population within the optical path, i.e. the metal atoms are all vaporized and, if they cross the optical path, measured by the spectrophotometer. However, the conversion efficiency is disrupted by two primary causes:
1. Loss of the analyte as a molecule, usually in combination with oxygen or chlorine, at a temperature below the vaporization temperature of the free metal analyte; and
2. Interference at the higher vaporization temperature of the analyte from molecules not containing the analyte, but absorbing in the measured range.
In the past, attempts were made to reduce interference by modifying the chemical matrix of the sample to be tested. Typically, a substituent was added to the solution containing the sample to stabilize the analyte, keeping it from being vaporized as an oxygen, chloride or other salt at a temperature below the vaporization, i.e. dissociation, step in the graphite furnace atomic absorption spectroscope procedure.
A poster session was presented on this topic in September, 1985, in Germany, for the P.R.O.C. Coll. Spec. Int. XXIV, Paper Tue. 077, by G. Schlemmer and B. Welz. In this work palladium and other platinum group elements were shown to stabilizing effect during graphite furnace atomic absorption spectroscopy on the metal analytes of arsenic, tellurium, mercury, thallium, lead and gold. The paper concludes that a mixture of palladium nitrate and magnesium nitrate can be used as a modifier for the more volatile elements of periodic groups IIIA to VIA of the periodic system. An increase in the vaporization temperature of several analytes is achieved and the risk of vapor phase interference is reduced accordingly.
Grobinski, Erler and Voellkopf authored an article entitled "Determination of Mercury With Zeeman Graphite Furnace AAS", appearing in Atomic Spectroscopy, Vol. 6, No. 4, July-August, 1985, which identified palladium as a matrix modifier for use in measuring mercury. The authors speculated that because palladium worked successfully as a matrix modifier only when pre-treated at 1000.degree. C. to 1100.degree. C., metallic palladium is formed at this high temperature and a stable compound is thereafter formed with the mercury though "amalgation". However, since gold and platinum do not work similarly to modify the matrix, they further concluded that simple amalgation is not the reason for the resulting matrix modification. The authors speculated that because palladium forms an oxide more readily than gold or platinum, such an oxide may be somehow required or involved in the reaction.
Palladium and ascorbic acid together have been shown to prevent interference normally present from perchloric acid in geological samples of river sediments tested for the presence of indium. Shan Xiao-Quan, Ni Zhe-Ming and Yuan Zhi-Neng wrote an article entitled "Determination of Indium in Minerals, River Sediments and Coal Fly Ash by Electrothermal Atomic Absorption Spectrometry With Palladium as a Matrix Modifier", Analytica Chemica Acta, 171, pp. 269-77, 1985, discussing this. Ascorbic acid and palladium are both disclosed as matrix modifiers for analyzing indium.
Although palladium has been known as a successful matrix modifier in the prior art, particularly for indium, thallium, tin, lead, arsenic, antimony, bismuth, selenium and tellurium in control samples, such disclosed methods are less valuable in testing true unknown samples where chemical interference cannot be standardized. Each constituent added as a solution in the graphite furnace contributes further contamination or, at least, the potential for contamination. When complex interactions, often times not fully understood even in controlled tests, are occurring in an unknown sample tested for a given analyte, such contaminants can effect the results by chemically interacting with the metallic analyte, or spectroscopically interfering by forming complexes which will absorb at the characteristic wavelength of the analyte in the vaporization temperature range of the analyte. Even the addition of reducing reagents in solution such as ascorbic acid, disclosed in the Shan Xiao-Quan article discussed above, introduce yet additional unknown constituents to the sample to be tested. Because of its reducing characteristics, ascorbic acid would have to be added as a separate solution in addition to the palladium in solution. Ascorbic acid is a powerful reducing agent and will precipitate palladium out of solution rapidly, making a pre-mixed solution impractical.
The Grobinski et al.'s method of reducing palladium at high temperatures, 1000.degree. C. to 1100.degree. C., is not practical because it requires a pre-heating step wherein the palladium is reduced by the hot graphite and cooled in the furnace before the sample is added to the furnace and the three-stage temperature scheme run. This is contrary to normal testing procedure and significantly increases the time needed for each analysis.
It is an object of the present invention to provide a matrix modifier which can be used to modify any of a large number of metal analytes in unknown samples without introducing additional contamination of a reducing agent in solution. It is another object of the present invention to provide a matrix modifier which can be used in unknown samples to test for a broad range of metal analytes and to improve the quantitative results of those tests. It is another object of the present invention to provide a matrix modifier which allows for a significant increase in the ash phase temperature but does not require a separate pretreatment step of the palladium, and which can be used to test a wide range of metallic analytes in unknown samples. It is yet another object to provide a method of modifying the matrix of a sample to increase the vaporization temperature of a metal analyte contained therein. It is yet an additional object of the present invention to provide a form of reduced palladium in the graphite furnace in contact with the metal analyte to obtain optimum effect of the palladium matrix modification properties. It is yet a further object of the present invention to provide an apparatus and method in which palladium is presented in an efficient form to metal constituents in unknown and known samples without contributing contamination of a reducing agent in solution. Further and additional objects will become apparent from the following discussion of the invention and its various embodiments.